1. Field of the Invention
The present invention relates to a compressor and more particularly to a method of operating a compressor for regulating the pressure in the discharge line.
2. Description of the Related Art
Such a compressor is commonly known in which the pressure in the discharge line, i.e., the discharging pressure, is kept equal to or higher than the minimum pressure needed for the load side and also the discharging pressure is kept from becoming higher than necessary so that energy saving operation is realized by opening/closing, i.e., by turning on/off, a suction regulating valve provided in the suction line (Japanese Patent Laid-open No. 4-159491). In the compressor, such control is carried out, when the discharge pressure (=the gas pressure supplied to the load side) becomes equal to or higher than a set upper limit of pressure, that the suction regulating valve is tuned off so that the supply of the suction gas to the compressor is stopped and the compressor is put into its unload operating state, whereas when the discharge pressure becomes equal to or lower than a set lower limit of pressure, the suction regulating valve is turned on so that a maximum suction quantity is supplied, i.e., the compressor is put into its load operating state, and thus, the discharge pressure is maintained within a predetermined range.
Further, in the compressor, in view of the fact that the life spans of such parts as the electromagnetic valve are shortened and reliability thereof is deteriorated when the suction regulating valve is frequently turned on/oft such control is carried out as to restrict the period of on/off operation within a set range of time (from .DELTA.t.sub.min to .DELTA.t.sub.max). Namely, the rate of change with respect to time t of the discharge pressure p (.DELTA.p/.DELTA.t) is detected while the compressor is driven and the set upper limit of pressure or the set lower limit of pressure is compensated for so that the aforesaid period is restricted within the set range of time. Accordingly, the larger the absolute value of the rate of change (.DELTA.p/.DELTA.t), the greater becomes the difference between the set upper and lower limits of pressure and, conversely speaking, the smaller the absolute value, the smaller becomes the difference between the set upper and lower limits of pressure.
There are problems with the above described conventional method of operating a compressor that it becomes necessary, when the load is changing, to compensate for the set upper and lower limits of pressure at all times and, especially when there are sudden load changes, that a period longer than an intended period must be set up to cope with such changes.
There is also such a problem with the above described operating method that an analog type pressure detector and an arithmetic unit for calculating the rate of change (.DELTA.p/.DELTA.t) are required.